1. Field of the Invention
The invention relates to a mask for projection exposure to be used for lithographic process in manufacturing semiconductor devices. Still more particularly, the invention relates to a mask for exposure using a phase shifter in order to improve the resolution.
2. Description of the Related Art
Fast functioning and high integration are two remarkable features of semiconductor devices developed in recent years. Finely defined precision patterns are required for pattern exposure systems in the production of semiconductor devices having these features and rays of light having a short wavelength such as far ultraviolet rays of light are widely used for light sources that are compatible with such patterns.
However, while an increased use of KrF excimer laser involving beams oscillating at a wavelength as short as 248 nm is expected for the light source of pattern exposure systems, there have not been developed any resists that can be effectively used with laser beams. Existing resists originally designed for use with g, i lines and electron beams are among inevitable alternatives that are currently available. Resists to be used for g and i lines are based on novolac resin having a large absorption coefficient for beams with a wavelength of 248 nm and, therefore, when KrF excimer laser beams are irradiated onto a surface of a resist film of this type, they are mostly absorbed by the film as they pass there without reaching the other side. Consequently, only a surface of the resist film is exposed to laser beam. Therefore, it is difficult to prepare a pattern having a large aspect ratio by using a resist for use with g or i lines.
A finely defined pattern may be formed by directly projecting electron beams on a resist film. However, this method of forming a pattern is accompanied by problems involving resist-heating, charging-up and the through-put of pattern production and, therefore, not suited for the production of patterns on a large scale. On the other hand, while the use of radioactive rays such as X-rays, electron beams or electrically charged corpuscular beams such as ion beams for producing patterns may be promising in terms of high resolution, aligners and resists that can be used with any of the above mentioned beams are still to be developed.
In short, any attempts to produce finely defined patterns by using unusual light sources face problems that may not be resolved easily. In view of this fact, efforts have been made recently to provide finely defined patterns by combining a conventional light source and a newly devised technique. One of such newly developed techniques is a phase shift method, with which a phase inverting layer is provided in areas of a resist film where rays of light are allowed to pass in order to prevent adverse effects of diffracted rays of light coming from adjacent patterns and thereby improve the fineness of the projected pattern.
Of variations of the phase shift method, a so called Levenson-type phase shift method utilizes a number of phase shifters arranged in an alternate manner in light transmission zones of a mask provided with opaque patterns. The phase of rays of light that have passed the phase shifters of the mask is inverted or shifted by 180.degree. relative to that of their counterparts that have passed through the areas of the mask where no phase sifters are disposed. In short, this method reduces interference of rays of light coming from adjacent patterns to enhance the resolution of the projected pattern by inverting the phase of rays of light passing through areas of the mask disposed adjacent to patterns.
Another variation of the phase shift method utilizes a phenomenon where dark areas are generated near the edges of a phase shifter. This technique can produce a very steep gradient in the intensity of projected light so that a finely defined space pattern may be formed when a negative type resist is used for wafer manufacturing processes. For instance, grooves having a width as small as 250 nm may be formed by using light beams having a wavelength of 365 nm for pattern alignment.
However, the above described phase shift method is not without problems. For one thing, while a Levenson-type phase shift mask shows an excellent resolution, it is subject to rigorous restrictions in terms of pattern arrangement. If a mask under consideration has a pattern arrangement as illustrated in FIG. 1 of the accompanying drawings, it may be understood that the phases of two adjacent patterns of the mask are inverted or shifted by 180.degree. relative to each other along broken line 11 but close to each other along broken line 12. Thus, such a mask may not be particularly excellent in resolution at locations where patterns show phases that are close to each other.
On the other hand, if the technique of utilizing dark areas generated near the edges of a phase shifter can effectively provide finely defined patterns, it cannot form large patterns. In other words, large patterns can be prepared only by using conventional opaque patterns. Therefore, while finely defined patterns may be produced for a phase shift mask by utilizing phase shifters, such a mask may not be able to operate satisfactorily because patterns having a large area of the mask are inevitably conventional light screen patterns having a relatively poor resolution.
Thus, on the one hand, a conventional Levenson-type phase shift mask is accompanied by a disadvantage that adjacent patterns can have phases that are identical with each other to the detriment of the achievable resolution. On the other hand, the technique of utilizing dark areas near the edges of a phase shifter does not address the problem of improving the resolution of a mask in the areas of the large patterns it comprises.